The RAM accelerator (or RAM cannon) is a device for accelerating projectiles to velocities vastly exceeding those possible using conventional guns. The concept (first demonstrated by Hertzberg et al., The Ram Accelerator: a New Chemical Method of Achieving Ultra-High Velocity, 37th Meeting of the Aeroballistic Range Association, Quebec, Canada, Sep. 9-12, 1986, and AIAA Journal, Vol. 26, No. 2, February 1988, pp. 195-203) uses a tube filled with reactive gas mixture consisting of fuel, oxidizer and, often, an inert gas as dilutant. The projectile is then injected into the tube at supersonic speed by using a conventional cannon. By careful design of the projectile and the tube, and appropriate choice of the gas combination, a system of shockwaves is established between the projectile and the tube, such that a chemical reaction takes place only at the predetermined location on the projectile. The shock wave from the bow of the projectile is reflected from the barrel at least once (more shock reflections may also be needed) and ideally impinges on the afterbody of the projectile. The passage through the two shocks heats and compresses the gas sufficiently, to initiate the desired chemical process (in this case supersonic combustion or oblique detonation) downstream of the reflected shock. The high pressure then acts on the projectile afterbody to accelerate it down the tube.
The specific, shock-induced combustion process that occurs is determined by the ambient gas mixture's composition and pressure, and the projectile's shape and velocity. For the oblique detonation to take place, the projectile must travel at a velocity exceeding the Chapman-Jouguet (C-J) velocity of the gas mixture (termed the super-detonative range). Detonation mode can be defined (following Pratt, D. T., Humphrey J. W. and Glenn D. F., Morphology of Standing Oblique Detonatiojm Waves, Journal of Propulsion, v.7, No. 5,. Sept-Oct. 1991, pages 837-45) as the process in which the shock is followed so closely by the supersonic combustion wave, that the two become strongly coupled and merge into a single detonation wave. It is feasible also that the supersonic combustion process follows the shock with sufficient delay (induction time) that it does not strongly affect the shock. The combustion process is thus decoupled from the shock. This is referred to as supersonic combustion, rather than detonation, although by some definitions the two are equivalent. Examples of such supersonic combustion modes can be found in Bogdanoff, D. W., Ram Accelerator Direct Space Launch System: New Concepts, J. Propulsion and Power, Vol. 8, No. 2, March-April 1992, pages 481-490, and Bruckner, A. P. and Knowlen, C., Overview of Ram Accelerator Technology, National Shock Wave Symposium, Institute of Fluid Sciences, Tohoku University, Sendai, Japan, 14-16 January 1993.
Other propulsive modes utilizing subsonic combustion have been more widely considered and analyzed. These include the mechanically and thermally choked modes discussed in the references mentioned hereinbefore. The thermally choked mode, where the combustion occurs in the wake of the blunt rear body segment thus maintaining a normal shock wave on the tapered tail section, is applicable to projectile speeds below the C-J velocity (termed the sub-detonative range). This mode is frequently used in the current ram accelerators operating in the sub-detonative range.
It is clear that the ignition process must be stationary relative to the projectile, and therefore that this mechanism is strongly dependent on the speed, shock strength and the distance between the projectile and the tube, as well as the reactive atmosphere's composition.
In all these systems, the accelerator barrel must be sufficiently narrow as to produce the reflected detonation waves. They may be called "internal propulsion" systems. To get away from the constraints of the tube geometry and thus the need for shock reflections, Jozef Rom proposed, in U.S. Pat. No. 4,932,306, corresponding to IL 82200, a ram accelerator which has a barrel that is wide enough not to produce reflected detonation waves, but detonation waves are produced by a shoulder portion in the form of a step, provided on the outer surface of the projectile. This is possible only if the gas properties and conditions are favorable, and the projectile's velocity is in the super-detonative range. Since the shoulder should be as small as possible, the leading edge shock on the projectile is assumed to provide a large part of the compression and heating of the gas mixture. This method allows, in essence, an external, tube independent propulsion mode. A simpler tube design would be possible both structurally and geometrically. The shoulder in the projectile geometry is, however, a drag and heat source and a way of keeping the projectile centered during the traverse must be assured. It is assumed that the guiding fins used in the original concept may not be practical because of the large distance betwen the projectile and the tube. Rom's system of propulsion may be called "external propulsion" system.
In U.S. Pat. No. 5,121,670 to Edward B. Fisher, a ram accelerator is described wherein a gas mixture is injected into the ram accelerator barrel at least at two points thereof, for example, one at the muzzle end and one at the inlet end, so as to produce an initial elevated pressure in the barrel before the projectile passes through the gas. The shock wave produced by the interaction of the two gas charges produces the desired elevated pressure. The shock and the compressed gas travel forward, with the projectile behind them. The shock reflected from the barrel ignites the gas mixture at the rear of the projectile.
The prior art ram accelerators are not fully satisfactory. For instance, premature ignition due to a shock pattern established by the forward parts of the projectile may occur and produce destructive deceleration of the projectile. Further, the known ram accelerator systems are not flexible insofar as the size of the barrel is concerned, for the barrel must have a small or a large diameter, depending on the system chosen. In the systems described by Hertzberg or by Fisher, the final gas pressures are very high, and thick and heavy barrels are required. In the system described by Rom, on the other hand, the constraint of the barrel is lifted, and the step in the projectile surface, which must have a significant height to generate the required strong shock, produces a large amount of unwanted drag, as well as a local heat problem.
It is a purpose of this invention to eliminate the drawbacks of the known ram accelerators.
It is another purpose of the invention to provide an accelerator system that can be used with a narrow barrel in internal propulsion ram mode, or with wide barrel for external propulsion, as needed.
It is a further purpose of the invention to provide a desirable control of the combustion process along the barrel.
It is a still further purpose of the invention to prevent premature ignition of the gas mixture due to shock patterns.
It is a still further purpose of the invention to anchor the reaction, either deflagration or detonation, to the jet.
It is a still further purpose of the invention to facilitate a method for truly external propulsion in the atmosphere, wherein the ambient air is utilized as the oxidizer.
It is a still further purpose of the invention to integrate the injection and gas supply mechanism used during acceleration in the device, either partially or in whole, with a jet steering system, to provide vehicle control during flight, and possibly during launch.
Other purposes and advantages of the invention will become apparent as the description proceeds.